1. Field of the Invention
The present invention relates to a vector control system of an induction motor, and more particularly, to a sensorless vector control system of an induction motor that is capable of estimating a magnetic flux and speed of an induction motor without using a speed measuring device.
2. Description of the Background Art
In general, thanks to its easy control, a DC motor has been for a long time used for a fixed speed and variable speed control apparatus. But the DC current is has shortcomings that its use of a predetermined time consumes a brush, which, thus, requires a maintenance and repairing.
In case of an induction motor, it is superior in the aspect of maintenance and repairing thanks to its firm structure. Especially, it""s low in price so that it has been widely used in the industrial field. But, the induction motor has been mainly used for a constant speed operation on account of its difficulty in controlling compared to a DC motor.
Recently, however, with the introduction of a vector control theory which is able to separately control a magnetic flux and a torque component by using a speed sensor, with the advent of a high speed power semiconductor device and with a development of a high performance microprocessor (Central Processing Unit or Digital Signal Processor), variable speed operation of the induction motor is possibly performed and the induction motor can be controlled beyond the level of the DC motor in terms of an efficiency of a speed control characteristic, so that the variable sped control field which has adopted the DC motor, growingly employs the induction motor in place of the DC motor.
In order to vector-control the induction motor, speed or magnetic flux information of the motor should be fedback from the induction motor, for which a speed information sensor or a magnetic flux sensor such as a tacho generator or a resolver or a pulse encoder is required.
However, since the sensors include an electronic circuit, the induction motor having the sensors is also restricted due to a use temperature range of the electronic circuit, and signal wiring between the speed sensor and the inverter incurs much expense.
And even though the speed sensors is possibly installed, since a coupling portion between the induction motor and the speed sensors are weak to an impact, the sensors are preferably avoided for use in terms of a facility reliability.
Thus, in order to solve such problems, researches for a sensorless vector control without a necessity of a speed sensor has been successively conducted.
Accordingly, recently, various speed estimation methods of the induction motor have been proposed with respect to the sensorless vector control without the speed sensor. Among them, researches are conducted on a method for directly estimating and controlling a magnetic flux by using a simultaneous differential equation of a model reference adaptive system (MRAS), a flux observer and a motor.
FIG. 1 is a schematic block diagram of a sensorless vector control system in accordance with a conventional art.
As shown in FIG. 1, a sensorless vector control system for receiving a power from a power supply unit 13 and driving an induction motor includes a speed controller for being fedback with a reference speed (xcfx89r*) and an estimation speed value ({circumflex over (xcfx89)}r) from an integration and proportional constant computing unit 20, operating them and outputting a reference torque component current (i1xcex2*), when the predetermined reference speed (xcfx89r*) and a reference magnetic flux component current (i1xcex1*) are given; a current to voltage command unit 10 for receiving the reference magnetic flux component current (i1xcex1*) and the reference torque component current (i1xcex2*) and outputting DC reference voltages (v1xcex1*, v1xcex2*); a DC to AC converter 11 for receiving the DC reference voltages (v1xcex1*, v1xcex2*) and outputting two phase reference AC voltages (v1d*, v1q*); a phase voltage converter 12 for receiving the two phase reference AC voltages (v1d*, v1q*) and three phase reference phase voltages (va*, vb*, vc*); an inverter 14 for receiving the three phase reference phase voltages (va*, vb*, vc*) and controlling an induction motor (IM); the induction motor 15 for receiving the three phase reference phase voltages (va*, vb*, vc*) from the inverter, to be driven; a current detector 16 for detecting a current flowing between the inverter and the induction motor and outputting detected phase currents (ia, ib, ic); a phase current converter 17 for receiving the detected phase currents (ia, ib, ic) and converting them into d-axis current (id) and q-axis current (iq); a magnetic flux operator 18 for receiving into d-axis current (id) and q-axis current (iq), receiving the two phase reference AC voltages (v1d*,v1q*), estimating two phase AC magnetic flux ({circumflex over (xcex)}2d,{circumflex over (xcex)}2q) and outputting them; an AC/DC converter 19 for receiving the estimated two phase AC magnetic flux ({circumflex over (xcex)}2d,{circumflex over (xcex)}2q), estimating a DC magnetic flux ({circumflex over (xcex)}2xcex1,{circumflex over (xcex)}xcex2) and outputting them; an integral/proportional constant computing unit 20 for estimating a speed by using {circumflex over (xcex)}2xcex2 of the estimated DC magnetic flux components and outputting it; a slip operator 23 for receiving a magnetic flux component current (i1xcex1*) and a torque component current (i1xcex2*), obtaining and outputting a slip; and an integrator 25 for receiving the slip and the estimated velocity ({circumflex over (xcfx89)}r), and integrating them to estimate an angle.
The operation of the sensorless vector control system constructed as described above will now be explained.
First, when the integral/proportional constant computing unit 20 receives a reference speed (xcfx89r*) from a user, operates and outputs a value. The speed controller 22 receives the value and outputs a torque component current (i1xcex2*).
Thereafter, the current/voltage command unit 10 outputs DC reference voltages (v1xcex1*, v1xcex2*) by using the magnetic flux component current (i1xcex1*) and the torque component current (i1xcex2*). The DC reference voltages (v1xcex1*, v1xcex2*) are is converted into two phase AC reference voltages (v1d*, v1q*) by the DC to AC converter 11.
Then, in order to drive an induction motor, the phase voltage converter 12 receives the two phase AC reference voltages (v1d*, v1q*) and outputs three phase reference phase voltages (va*, vb*, vc*), and the inverter 14 drives the induction motor by using power provided from a power supplier and the three phase reference phase voltages (va*, vb*, vc*).
An estimated velocity ({circumflex over (xcfx89)}r) and an estimated angle ({circumflex over (xcex8)}e), are obtained as follows.
A current flowing between the inverter 14 and the induction motor 15 is detected to obtain three phase currents (ia, ib, ic). The three phase currents (ia, ib, ic) are converted into two phase d-axis current (id) and q-axis current (iq), which are easily controlled, and outputted by the phase current converter 17.
The magnetic flux operator 18 receives the output values (v1d*, v1q*) of the DC/AC converter 11 and the d-axis current (id) and the q-axis current (iq), and estimates two phase AC magnetic flux to estimate two phase AC magnetic flux ({circumflex over (xcex)}2d,{circumflex over (xcex)}2q).
The AC/DC converter 19 converts the two phase AC magnetic flux ({circumflex over (xcex)}2d,{circumflex over (xcex)}2q) to two phase DC magnetic flux ({circumflex over (xcex)}2xcex1,{circumflex over (xcex)}2xcex2) which can be conveniently controlled, and then the integral/proportional constant computing unit operates and obtains an estimated velocity ({circumflex over (xcfx89)}r) by using the component {circumflex over (80 )}2xcex2 of the two phase DC magnetic flux components.
The estimated velocity ({circumflex over (xcfx89)}r) and the output of the slip operator 23 are added to obtain an estimated angular velocity ({circumflex over (xcfx89)}e) and the integrator 25 estimates an angle ({circumflex over (xcex8)}e) required for reference frame conversion by using the estimated angular velocity ({circumflex over (xcfx89)}e).
The conventional sensorless vector control system described above, however, has many problems
For example, first, since the current/voltage command unit does not include a differential term for the magnetic flux, only the normal state is considered, and in a transient state, it is not possible to perform an instantaneous torque controlling.
Secondly, in order to obtain the estimated velocity ({circumflex over (xcfx89)}r), a proportional constant and an integral constant values should be accurately computed to be used by the integration and proportional constant computing unit 20, which are very difficult because the proportional constant and the integral constant values are different for every motor and they are hardly obtained substantially.
Thirdly, as described above, in case that the speed of a motor is controlled by the conventional sensorless vector control system, when it is driven with a low speed algorithm, it adopts a method in which a high frequency voltage or current is added to a primary wave voltage to search an absolute position of a magnetic flux of the motor, This method is effect at a low speed, but not possibly used at a high speed. Meanwhile, in case of the high speed algorithm, when the speed of the motor is controlled, the algorithm is effective at a high speed but its implementation is very difficult at a low speed. Consequently, it fails to cover the whole speed range.
Therefore, an object of the present invention is to provide a sensorless vector control system of an induction motor that is capable of being stably operated in an every speed range and capable of precisely controlling a speed and a torque.
Another object of the present invention is to provide a sensorless vector control system of an induction motor that is capable of automatically compensating a constant variation of a motor and a voltage error at a low speed range.
Still another object of the present invention is to provide an easily realizable algorithm by reducing a dependence on a parameter of an induction motor and an arithmetic operation without using a high performance operational unit.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a sensorless vector control system of an induction motor in which power is supplied from a power supply unit to drive an induction motor, including: a magnetic flux and speed controlling unit for receiving a predetermined command value and generating two phase voltages of DC component; a first coordinate converting unit for converting the two phase voltages of DC component into three phase voltages of AC component; an inverter for receiving the three phase voltages of AC component and driving an induction motor; a current detecting unit for receiving the three phase power of AC component flowing between the inverter and the induction motor, and detecting and outputting three phase currents of AC component; a second reference frame converting unit for receiving the three phase currents of AC component, and converting and outputting two phase currents of DC component; a magnetic flux and speed estimating unit for receiving the two phase voltage of DC component and the two phase currents of DC component, estimating a magnetic flux and speed required for a vector control; and a primary resistance estimating unit for receiving the two phase voltages of DC component, the two phase currents of DC component and the magnetic flux and speed estimated values, estimating a primary resistance and outputting it.
To achieve the above objects, there is further provided a sensorless vector control method of an induction motor in which power is supplied from a power supply unit to drive an induction motor, including the steps of: receiving a predetermined command value and generating two phase voltages of DC component; converting the two phase voltages of DC component into three phase voltages to drive an induction motor; detecting three phase power of AC component flowing at the induction motor when the induction motor is driven and outputting three phase currents of AC component; converting the three phase currents of AC component into two phase currents of DC component; receiving the voltages and currents of DC components and outputting a magnetic flux and speed estimated value by using an algorithm required for vector control by a magnetic flux and speed estimated values; and receiving the voltages and currents of DC component and the magnetic flux and speed estimated values and estimating a primary resistance by a primary resistance estimator.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.